1. Field of the Invention
The present invention relates to a separator for a fuel cell which is mainly used as a cell for an electric vechicle, and also to a method of producing the separator, and more particularly to a separator for a fuel cell of the solid polymer electrolyte type or the phosphoric acid type, and also to a method of producing the separator. In a fuel cell of such a type, a unit cell which is a unit constituting the cell is configured by: sandwiching a gas diffusion electrode having a sandwich structure wherein an electrolyte membrane configured by an ion exchange membrane, between an anode and a cathode; sandwiching the gas diffusion electrode between separators; and forming fuel gas passages and oxidant gas passages between the separators, and the anode and the cathode.
2. Description of the Prior Art
In a fuel cell, a fuel gas containing hydrogen is supplied to an anode, and an oxidant gas containing oxygen is supplied to a cathode, so that, in the anode and the cathode, electrochemical reactions indicated by the formulae:
xe2x80x83H2xe2x86x922Hxe2x88x92+2exe2x88x92xe2x80x83xe2x80x83(1)
(xc2xd)O2+2Hxe2x88x92+2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(2)
occur, and, in the whole of the cell, an electrochemical reaction indicated by the formula:
H2+(xc2xd)O2xe2x86x92H2Oxe2x80x83xe2x80x83(3)
proceeds. The chemical energy of the fuel is directly converted into electrical energy, with the result that the cell can exert a predetermined performance.
A separator for a fuel cell of the solid polymer electrolyte type or the phosphoric acid type that is a kind of fuel cell in which such energy conversion is conducted is desirably gas-impermeable, and also is made of an electrically conductive material. As a material meeting the requirements, conventionally, an electrically conductive resin is used. An electrically conductive resin is a complex which is configured by bonding graphite (carbon) powder by means of a thermosetting resin such as phenol resin, or a so-called bondcarbon (resin-bonded carbon) compound. A separator for a fuel cell is configured by forming such a bondcarbon compound into a predetermined shape.
Conventionally, a separator for a fuel cell having a predetermined shape is formed by using such a bondcarbon compound in the following manner. With respect to the composition ratio of a thermosetting resin such as phenol resin and graphite powder, 25 to 40 wt. % of the thermosetting resin is used as an adequate content in consideration of moldability and workability of the bondcarbon compound.
In a conventional separator for a fuel cell which is configured by using a bondcarbon compound of such composition ratios, the content of a thermosetting resin serving as an electrically insulating material is large, and hence the conductivity of the separator itself is lowered so that the electrical resistance is increased. This is not preferable from the viewpoint of the performance of a fuel cell.
In order to improve the conductivity of a separator for a fuel cell which is configured by using a bondcarbon compound, it is contemplated that the content of a thermosetting resin be reduced as far as possible, and graphite powder of excellent crystallinity and having less impurities such as ash is selectively used. When the content of a thermosetting resin is reduced, however, elongation and fluidity of the bondcarbon compound during a molding process are lowered, and moldability is impaired. In order to improve the conductivity of a separator, furthermore, it is required to use graphite powder of excellent crystallinity. However, graphite powder of excellent crystallinity is usually poor in wettability and bondability to a resin. When graphite powder of excellent crystallinity is used, therefore, a larger resin content is required. As a result, a uniform separator is obtained more hardly as the resin content is smaller.
This will be described in more detail. With respect to a volume resistivity serving as an index of the conductivity of a separator which largely affects the performance of a fuel cell, it is requested to develop a separator having a volume resistivity of 1xc3x9710xe2x88x922 xcexa9xc2x7cm or lower. In order to obtain a separator of a volume resistivity of such a degree, the resin content in the composition ratio with respect to graphite must be reduced to 15 wt. % or less. When the resin content is reduced to 15 wt. % or less, however, elongation and fluidity of the bondcarbon compound are lowered, and, when the resin content is smaller than 3 wt. %, the bondcarbon compound has substantially no fluidity. In the case where a bondcarbon compound of a resin content of 15 wt. % or less is used, even when the compound is charged into a heated mold, therefore, the compound hardly extends to every corners of the mold because the lack of the resin content causes the molten compound to exert insufficient fluidity. As a result, even when a molding pressure higher than a usual pressure or 5 to 10 MPa is applied, uneven molding easily occurs depending on the charging state of the compound, and a molded member (separator) which is normal in shape cannot be sometimes obtained.
The present invention has been conducted in view of the above-mentioned circumstances. It is an object of the invention to provide a separator for a fuel cell which can be formed into a uniform and predetermined shape while good conductivity is ensured by reducing the resin content so as to suppress the volume resistivity to 1xc3x9710xe2x88x922 xcexa9xc2x7cm or lower.
It is another object of the invention to provide a method of producing a separator for a fuel cell wherein, even when a molding material in which the resin content is small, and which is therefore low in elongation and fludity is used, the molding material can extend to every corner of a mold so that the separator having a uniform and correct shape that is free from molding unevenness, and good conductivity can be surely produced.
In order to attain the objects, the separator for a fuel cell of the invention is a separator for a fuel cell consisting of a complex which is configured by bonding graphite powder by means of a thermosetting resin, and characterized in that, in the complex, a composition ratio of the graphite powder is set to 85 to 97 wt. %, a composition ratio of the thermosetting resin is set to 3 to 15 wt. %, an average particle diameter of the graphite powder is set to a range of 15 to 125 xcexcm, and the complex is molded at a pressure of 10 to 100 MPa.
Preferably, the composition ratio of the thermosetting resin in the complex is set to a range of 4 to 9 wt. %, and the average particle diameter of the graphite powder is set to a range of 40 to 100 xcexcm. Preferably, the molding pressure of the complex is set to a range of 20 to 50 MPa.
In order to meet the above-mentioned demands for development, intensive studies on a separator for a fuel cell which is configured by using a bondcarbon compound, and finally found that the volume resistivity serving as an element which largely affects the performance of a fuel cell is determined not only by the composition ratios of a resin and graphite powder, but also by the average diameter of the graphite powder, and the molding pressure, and that the size of the average diameter of the graphite powder is closely related not only to the volume resistivity, but also to the fluidity, moldability, and strength of the compound. Based on this finding, the composition ratios of a resin and graphite powder, the average diameter of the graphite powder, and the molding pressure have been respectively set to the above-mentioned ranges, thereby completing the invention.
According to the thus configured invention, as the graphite powder which is the one composition of the complex and which largely affects the volume resistivity, graphite powder in which the average diameter is set to a range of 15 to 125 xcexcm, preferably, 40 to 100 xcexcm is used, the thermosetting resin which is the other composition of the complex, and which largely affects fluidity, modability, and strength is reduced to 3 to 15 wt. %, and a high molding pressure of 10 to 100 MPa is applied to the complex to increase the mold density, whereby the volume resistivity of the complex, and therefore that of the separator are lowered, so that elongation and fluidity of the complex serving as a molding material can be enhanced while improving the conductivity of the complex. As a result, the invention attains an effect that, even when graphite powder of excellent crystallinity is used, the compound can surely extend to every corner of a mold so that a uniform separator of a desired shape can be surely obtained, and the performance of a fuel cell can be remarkably improved.
The method of producing a separator for a fuel cell according to the invention is a method of producing a separator for a solid polymer electrolyte type fuel cell configured by molding a complex in which composition ratios are set to 85 to 97 wt. % of graphite powder, and 3 to 15 wt. % of a thermosetting resin, and an average diameter of the graphite powder is set to a range of 15 to 125 xcexcm, and characterized in that the complex is previously cold-molded into a shape similar to a final molded shape, the preliminary molded member is then placed in a mold, and the preliminary molded member is molded into the final shape by applying a pressure of 10 to 100 MPa.
Preferably, the composition ratio of the thermosetting resin in the complex is set to a range of 4 to 9 wt. %, and the average particle diameter of the graphite powder is set to a range of 40 to 100 xcexcm. Preferably, the molding pressure of the complex is set to a range of 20 to 50 MPa.
The shape similar to a final molded shape means that the dimensions other than those in the direction of the molding pressure are similar to corresponding ones of the final molded member. Preferably, dimensions of the preliminary molded member in the direction of the molding pressure are set to be about 1.0 to about 2.0 times dimensions of the final molded member. When such a preliminary molded member is used, the mold density and the volume resistivity can be further improved.
According to the production method of the invention having the above-described molding means, the two-step molding is employed wherein a complex (bondcarbon compound) in which elongation and fluidity are lowered and moldability is impaired by reduction of the resin content which is performed in order to lower the volume resistivity and enhance the conductivity is previously cold-molded into a shape similar to the final molded shape, and the preliminary molded member is placed in a mold and then molded into the final shape by applying a high molding pressure of 10 to 100 MPa. Even when a complex (molding material) which is low in elongation and fluidity is used, therefore, the compound can surely extend to every corner of the mold so as to eliminate molding unevenness, and be uniformly charged into the mold so as to increase the mold density. As a result, the conductivity can be made very excellent by reduction of the volume resistivity, and furthermore it is possible to surely obtain a uniform separator which is correct also in shape.
In the separator for a fuel cell and the method of producing a separator for a fuel cell according to the invention, particularly, it is preferable to set the molding pressure of the complex to a range of 20 to 50 MPa. The molding pressure, and the mold density and the volume resistivity have the correlation shown in FIG. 5. At a molding pressure in a range of 5 to 10 MPa which is usually used in the conventional art, both the mold density and the volume resistivity fail to reach values which are required in a separator for a fuel cell. By contrast, when the molding pressure is set to a range of 20 to 50 MPa, both the mold density and the volume resistivity are stabilized to substantially constant values. When the molding pressure is set to 20 MPa at the minimum, it is possible to obtain a separator of good conductivity.
In the method of producing a separator for a fuel cell according to the invention, a final molding temperature of the complex is preferably set to a range of 150 to 170xc2x0 C. When molding is conducted in this temperature range, the moldability such as elongation and fluidity in molding of the complex in which elongation and fluidity are small because of the reduced resin content can be compensated so that a separator of a predetermined shape can be obtained.
As the thermosetting resin which is useful in the invention, phenol resin which is excellent in wettability with respect to graphite powder may be most preferably used. Alternatively, any other resin such as polycarbodiimide resin, epoxy resin, furfuryl alcohol resin, urea resin, melamine resin, unsaturated polyester resin, or alkyd resin may be used as far as the resin causes a thermosetting reaction when the resin is heated, and is stable against the operating temperature of the fuel cell and components of the supplied gasses.
As the graphite powder which is useful in the invention, powder of graphite of any kind, including natural graphite, artificial graphite, carbon black, kish graphite, and expanded graphite may be used. In consideration of conditions such as cost, the kind can be arbitrarily selected. In the case where expanded graphite is used, particularly, a layer structure is formed by expanding the volume of the graphite as a result of heating. When the molding pressure is applied, layers can twine together to be firmly bonded to one another. Therefore, expanded graphite is effective in a complex in which the ratio of a thermosetting resin is to be reduced.
Other objects and effects of the invention will be clarified in embodiments which will be described below.